Crossflow heat exchanger



Feb. 22, 1949. P. A. PITT CROSS-FLOW HEAT EXCHANGER Filed 001;. 26, 19442 Sheets-Sheet 1 JQ JUL 1 /77;

INVENTOR.

ATTORNEY.

22, p PITT CROSS-FLOW HEAT EXCHANGER 2 Sheets-Sheet 2 Filed Oct. 26,1944 INVENTOR.

PAUL A. PITT ATTORNEY Patented Feb. 22, 1949 CROSSFLOW HEAT EXCHANGERPaul A. Pitt, San Diego, Calif., assignor to Solar Aircraft Company, SanDiego, Calif., a corporation of California I Application October 26,1944, Serial No. 560,369

Claims. (Cl. 257-245) This invention relates to heat exchangers of thecross-flowv type, in which two fluids between which heat transfer is totake place flow in directions substantially at right angles to eachother. More particularly, it relates to that type of cross-flow heatexchanger in which a series of generally fiat thin metal sheets are sosecured together at their edges as to define the two sets of passages.

The invention is particularly useful on aircraft for heating air by theexhaust gas from the engine where the temperature differential betweenthe exhaust gas and the air to be heated is frequently very great. Thusthe temperature of the air at high altitudes is frequently 60 F. or morebelow zero whereas the exhaust gas may be 1800" F. or more above zero.As a result, different parts of the heat exchanger, only a few inchesapart, may acquire temperatures differing by as much as 1200 F. Becauseof the resultant different expansions and contractions oi relativelyclosely spaced parts of the heat exchange structure, severe stresses aredeveloped that eventually result in the break-down of a structure thatwould be adequate when subject to less severe temperature differentials.

A general object of the present invention is to increase the life ofheat exchangers of the type mentioned.

Another object is to reduce the temperature difierentials between insideand outside portions of a heat exchanger of the type described andthereby reduce differential expansions between the parts.

Another object is to prevent vibration of the plates or sheets of a heatexchanger.

Still another object is to provide a structure that permits the sheetsto bend in response to difierential expansion caused by unavoidabletemperature differentials.

Ordinarily there is a marked difference in the temperatures of theoutside sheets of a heat exchanger and the interior sheets because theinterior sheets are exposed only to the cooling efiect of the coolingfluid that is flowing through the heat exchanger whereas the outsideplates are exposed not only to the cooling medium flowing therethroughbut also lose heat by radiation. In accordance with the presentinvention, I reduce the difierential temperature between the inner andouter sheets of the exchanger by throttling the flow of cooling fluidthrough the outer passages so that less heat is absorbed from the outerplates by the cooling fluid, to compensate for. the heat lost from thoseplates by radiation.

I reduce vibration of the plates, in accordance with the presentinvention, by providing corrugations or beads extending in bothdirections across the plates and either in contact with or welded to thecorrugations of the adjoining plates so as to reduce the free areas ofthe plates to stiffen them and reduce opportunity for drumming. I alsoso form the corrugations that they facilitate gradual bending of thesheets in response to differential expansion of diflerent portions ofthe sheets so as to reduce the overall stresses due to unequal expansionand which ordinarily produce failure of the structure within arelatively short time.

Various more specific objects and features of the invention will becomeapparent from the following detailed description of a specific em-'bodiment of the invention, as illustrated in the drawing:

-. In the drawing:

Fig. 1 is a side elevation view of the core of a heat exchanger inaccordance with the invention, portions of the structure being brokenaway;

Fig. 2 is a cross section of the core shown in Fig. 1, with the outercasing shown in section and a portion of the core broken away to showhow it appears in a diiferent plane;

Fig. 3 is a perspective view, with parts broken away, showing the cornerconstruction of the heat exchanger; and

Fig. 4 is a plan view of one of the sheet units that make up the core.

Referring to Fig. 1, there is disclosed a crossflow heat exchangerhaving a core of special construction in accordance with the invention,which core is adapted to be contained in a casing. The particularexchanger illustrated is of approximately square cross section and is ofgreater length than width. It is intended for use for heating air fromthe exhaust gas of an airplane engine and, as usually employed, theexhaust gas is passed longitudinally through the exchanger and the airis passed through it transversely.

The core of the exchanger consists of a plural; ity of similar units I3,each of which constitutes a conduit extending longitudinally of theexchanger and, in this instance, conveying hot exhaust gas. Each unit l3consists of two formed sheets l5 and IS in spaced relation over themajor portion of their areas but brought together and welded at theirlateral edges I I, and having opposite corrugations II spaced inter- 3mediate the edges I4, the corrugations on the two sheets I5 and I6 beingalso welded together so that three distinct longitudinal passages I8, I8and I8 are formed by each unit I3.

The lateral edges I4 of eacl. unit I3 are flattened at the corners, asshown in Fig. 3, to form vertical corner edges 22 on each unit.

The lower sheet I5 of each intermediate unit I3 is connected to theupper sheet I6 of the next unit therebelow at each end by a clip asindicated in Figs. 1 and 3, and each intermediate cross-flow passage 2|is defined by the sheet I5 ofone unit I3 and the sheet I6 of the nextadjacent unit I3.

The joinder of the horizontal end edges of the units by the clips 20, asdescribed, maintains the corner edges 22 of the various units inalignment and they are welded together to maintain a separation of thecross-flow passages 2| from the longitudinal passages I8.

To conduct fluid to and from the longitudinal passages I8, transitionmembers 24 and 25 are provided at the opposite ends of the core. Thetransition member 24 is rectangular in crosssection at its inner end andfits in and is welded to arectangular collar 2 which in turn is weldedto the left end edges of the top wall I6 of the top unit I3 and of thebottom wall I5 of the bottom unit I3. The other two edges of the collar24I are welded to the vertical edges of the core, which are constitutedby the aligned corner edges 22 of the various units I3. The outer end ofthe square collar 2 is turned out to form a flange 242. The transitionmember 25 is joined to a square collar 25I having an outwardly extendingflange 252, and the collar 25I is joined to the right end of the core ofthe exchanger in the same manner that the collar 24I is joined to theleft end thereof.

Upper and lower casing walls 26 and 21, respectively, are provided aboveand below the top and bottom units I3, to provide a pair of outertransverse passages 2Ia for conveying fluid past the outer sides of theupper and lower units I3. These casing walls 26 and 21 are secured attheir ends to the transition members 24 and 25 and are provided withgroove-defining attachment flanges 28 for receiving conduits whichconvey fluid to and from the cross-flow passages. As previously stated,ordinarily the fluid flowing through the cross passages 2| and 2Ia willbe the cooler fluid, such as air, whereas the fluid flowing through thelongitudinal passages I8 would be exhaust gas at a high temperature.

Although the outer cross-flow passages 2-Ia are highly desirable for thereason that they provide additional capacity for the exchanger, I havefound that as a result of the heat loss to the air flowing throughpassages 2Ia, in addition to the heat loss by radiation to and throughthe outer casing wall 26, which is exposed to the surrounding air, theouter sheet I6 of the top unit I3 may run at a much lower temperaturethan the inner sheet I5. Likewise the lower sheet I5 of the bottom unitI3 may run at a lower temperature than the top sheet of that unit. Thesedifferences in temperature produce differential expansions in the twowalls of the outer units I3 which materially shorten their lives.- Ihave found that this inequality in temperature may be largely correctedby providing baffles 29 at one end of each passage 2Ia. These baffles 29are preferably provided at the inlet end although this is not essential.The bailies are so proportioned by test as to produce substantially.equal temperatures in the two sheets I5 and I6 forming the outer unitsI3.

As already described, the sheets I5 and I6 of each unit I3 are weldedtogether at their edges I4 and at the beads or corrugations II so thatthe lateral dimensions of the sheets between points of joinder arerelatively small, thereby reducing the opportunity for free vibration ofthe plates.

.To further stiffen and support the plates, they are provided withexternally projecting transverse corrugations 30, and the corrugation 30of each plate I5 of one unit I3 contacts the corrugation 30 of theadjacent plate I6 in the next adjacent unit I3. This construction ismost clearly shown in Fig. 1. It has the advantage of providing mutualsupport between the plates at relatively closely spaced pointslongitudinally so as to reduce the unsupported length of any section ofeach sheet.

It is found in practice that substantial temperature difierences mayobtain between the edge portions I4 and the corrugated portions H ofeach unit I3, relative to the unsupported areas intermediate the joinedportions I4 and I1. These differences in temperature result in unequalexpansion, which in older designs have within the undulate portion. Itwill be observed that both sheets I5 and I5 of each unit I3, and of thedifferent units I3, are all curved similarly, so that the passages areof substantially the same cross sectional area at all points. Anexception is that the cross-flow passages 2I are larger adjacent thecorrugations I'I. However, these enlarged portions are beneficial topromote turbulence of the air flowing through the passages 2 I.

The heat exchanger described is preferably constructed by first formingthe units I3 individually by seam-welding the edges I4, and thecorrugations II together. ments I3 are stacked together and flame or arewelded together at the ends 20. Following assembly of the core, alongthe vertical corner edges 22, along the end edges of the top wall ft ofthe top unit I3, and along the end edges of the bottom wall I5 of thebottom unit I3, the transition members 24 and 25 may be welded to theends of the core, after which the upper and lower casing walls 26 and 21can be welded or joined in other manner to the transition members 24 and25.

Although for the purpose of explaining the invention a particularembodiment thereof has been described in detail, various departures fromthe exact construction shown can be made without departing from theinvention which is to be limited only to the extent set forth in theappended claims.

I claim:

1. In a heat exchanger of the cross-flow type having a first set ofgenerally flat parallel passages extending in one direction forconducting a first fluid that is above ambient temperature and having asecond set of generally flat parallel passages interleaved with thefirst set and extending in a direction substantially at right angles toThereafter, the elesaid one direction for conducting a second, cooler,fluid, the construction comprising: a plurality of generally fiat sheetsin generally parallel spaced relation with successive pairs of sheetsjoined directly together along one pair of opposite edges and definingfiat conduit units constituting the sole essential structure formingsaid first set of passages, and the adjacent sheets of each adjacentpair of said conduit units being joined directly together along theirother pairs of opposite edges for defining all of the interior passagesof said second set of passages; auxiliary casing walls parallel and inoutwardly spaced relation to the outermost of said sheets for definingtherewith the outermost passages of said second set of passages; andbaiile means extending across said outermost passages at one end thereoffor restricting the velocity of flow of said second cooler fluid throughsaid outermost passages more than through the interior passages of saidsecond set to reduce the temperature difi erential of said outer sheetsrelative to the remaining sheets.

2. In a heat exchanger of the cross-flow type having a first set ofgenerally flat parallel passages extending in one direction forconducting a first fluid that is above ambient tem erature and having asecond set of generally flat parallel passages inter-leaved with thefirst set and extending in a direction substantially at right angles tosaid one direction for conducting a second, cooler, fluid, theconstruction comprising: a plurality of generally fiat sheets ingenerally parallel spaced relation with successive pairs of sheetsjoined directly together along one pair of opposite edges and definingflat conduit units constituting the sole essential structure formingsaid first set of passages, and the adjacent sheets of each adjacentpair of said conduit units being joined directly together along theirother pairs of opposite edges for defining all of the interior passagesof said second set of passages, said sheets having straight corrugationsextending longitudinally of and projecting into said first passages andthe corrugations the full length of each pair of sheets that define oneof said first passages merging and being welded together, whereby saidsheets are reinforced.

3. In a heat exchanger of the cross-flow type having a first set ofgenerally flat parallel passages extending in one direction forconducting a first fluid that is above ambient temperature and having asecond set of generally flat parallel passages inter-leaved with thefirst set and extending in a direction substantially at right angles tosaid one direction for conducting a second, cooler, fluid, theconstruction comprising: a plurality' of generally fiat sheets ingenerally parallel spaced relation with successive pairs of sheetsjoined directly together along one pair of opposite edges and definingflat conduit units constituting the sole essential structure formingsaid first set of passages, and the adjacent sheets of each adjacentpair of said conduit units being joined directly together along theirvother pairs of opposite edges for defining all of the interior passagesof said second set of passages, each pair of adjacent sheets thatdefines a passage of said first set being longitudinally straight attheir joined edges and being longitudinally undulate intermediate theirjoined edges, whereby the intermediate portions can yield" to compensatefor relative expansion and contraction of the edge portions with respectto the intermediate portions.

4. A heat exchanger as defined in claim 3, in which the two sheets ofeach pair defining one of said second passages have juxtaposedcorrugations extending longitudinally of and projecting into the secondpassages into contact with each other.

5. In a heatexchanger of the cross-flow type having a first set ofgenerally flat parallel passages extending in one direction forconducting a first fluid that is above ambient temperature and having asecond set of generally flat parallel passages inter-leaved with thefirst set and extending in a direction substantially at right angles tosaid one direction for conducting a second, cooler, fluid, theconstruction comprising: a pluralityof generally flat sheets ingenerally parallel spaced relation so joined together at their edgesthat they constitute the sole essential structure defining said firstset of passages and all the interior passages of said second set, eachpair of adjacent sheets that defines a passage of said first set beinglongitudinally straight at their joined edges and having straightlongitudinal corrugations welded together and spaced from each other andfrom said joined edges, the two sheets of each pair defining one of saidsecond passages having juxtaposed straight corrugations extendinglongitudinally of and projecting into the second passage into contactwith each other, said sheets being longitudinally undulate intermediatetheir joined edges and said joined corrugations and intermediate thecorrugations projecting into said second passages, whereby theunsupported portions of said sheets can yield to compensate for relativeexpansion and contraction of said different portions of the sheets.

PAUL A. PITT.

nernnanons crrnn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 849,878 Zivi Apr. 9, 19071,472,954 Behringer Nov. 6, 1923 1,734,962 Clarke Nov. 12, 19291,751,725 Cross Mar. 25, 1930 1,775,103 Hume Sept. 9, 1930

